aldar

In a sentence

• Það hefur verið mikil þróun á þessari öld.

  There has been significant development in this century.

  Here, "öld" is used in the singular dative form ("öld" -> "öldin" -> "á öld") to denote the time period of a century.

• Hann er að rannsaka sögu nítjándu aldar.

  He is studying the history of the nineteenth century.

  In this sentence, "aldar" appears in the genitive singular form to indicate possession or relation, pointing out the particular century the history pertains to.

• Við skulum skoða áhrif þessarar aldar á samfélag okkar.

  Let us observe the impact of this century on our society.

  Here, the word "aldar" is employed in the genitive singular form as a descriptor for belonging or impact related to the current century.